This invention relates to the treatment of waste water. The invention finds particular utility in the treatment of waste water from a domestic laundry process which water contains dyestuffs.
Waste water is the aqueous residue which is usually discarded following domestic or industrial processes eg, for the treatment of fabrics or textiles.
Water pollution is a major problem world-wide. One aspect of the problem is the discoloration of waste water due to the presence of organic dyes from domestic processes (eg, processes involving laundry and foods) and industrial processes (eg, in the textile, food, beverages, pharmaceutical, cosmetics, plastics, paper, leather, printing inks, paint, varnish, lacquer and wood stain industries).
Due to the high extinction coefficients of the dyes, only very small quantities (eg, a few parts per million) are required to give discoloration. The dyes present are generally chemically very stable making them difficult to remove. Common methods to remove such pollution are filtration, reverse osmosis and flocculation. However, these treatments are expensive (see P. Pitter and P. Chudoba, Biodegradability of Organic Substances in the Aquatic Environment, CRC Press: Boca Raton, 1994).
WO 94/16056 discloses a process for the destruction of toxic waste materials, such as chlorinated hydrocarbons by the use of free-radical chemistry. In that patent the free radicals are generated by a free radical generating catalyst (generally a white-rot fungus peroxidase) in the presence of a mediator compound and a reductant.
WO 98/57895 relates to the oxidation of sewage by a radical process maintained by the incorporation of a radical starter, a catalyst and oxygen into the sewage flow.
U.S. Pat. No. 4,246,105 discloses a process for purifying water using free peroxygen radicals generated by means of radiation or electrical means.
It is known that hydroxy (HO.) or alkoxy (RO.) radicals produced via Fenton chemistry can decompose pollutants (see, for example, Halmann, M. Photodegradation of water pollutants; CRC Press: Boca Raton, Fla., 1996). However, these systems require the presence of transition metals in the water, which can be difficult to remove after treatment.
There remains a need for systems for the treatment of waste water which do not involve the addition of transition metal compounds, or other undesirable compounds, to the waste water. It is an object of the invention to provide such systems.
Radical initiators, which decompose to form radicals on exposure to light or to heat, are themselves well-known and are used in the unrelated fields of polymerisation, polymer cross-linking and curing reactions in materials.
According to the invention, there is provided a method for the treatment of waste water by the generation of radicals in the waste water, which comprises applying to the waste water at least one radical photo- or thermal-initiator which is capable of providing radicals in the waste water, wherein the substance is capable of decomposing by a unimolecular (bond-cleavage) mechanism to form a carbon-centred radical on exposure to light or to heat.
Carbon-centred radicals are those in which the unpaired electron in the radical is most stable at or near to a carbon atom. These differ from the oxygen and peroxygen radicals described above.
The method of the invention has been found to be capable of removing the colour from waste water which is discoloured, particularly where the discoloration of the waste water is caused by one or more organic dyes. It can also kill certain bacteria present in the waste water.
The method of the invention can decolourise waste water relatively rapidly and does not add any compounds to the waste water which may cause it to become coloured (such as transition metal ions). Also, the decomposition products which are formed from the substance can be more readily biodegradable than the residual products of conventional compounds for treating waste water.